Modern internetworking and related technologies allow beneficial communications, file sharing and other useful applications. Electronic mail (“email”), for instance, provides a virtually cost-free medium for communication with others. The low cost of email communications can achieve even greater economical benefit by sending a particular email message to multiple recipients. But email, as a communication medium, is subjected to misuse. For example, unscrupulous advertisers can exploit email's advantage by sending unwanted, unsolicited, and annoying email messages. Such troublesome email messages comprise spam and there are many approaches to combat email spam.
Internet Protocol (IP) telephony is another fast growing application that provides low cost communication with others. The advantages of IP telephony typically relate to feature availability and cost savings. As IP telephony is increasingly used in open network environments, users may receive more unsolicited and/or unwanted calls and messages.
Unsolicited and/or unwanted calls over IP telephony are known as “Spam over IP telephony” (SPIT). SPIT deals with unwanted voice calls before the calls are answered, and once the calls are answered, it is too late to categorize the calls as SPIT.
On the other hand, unsolicited commercial voice mail messages, or voice mail spam, present different challenges as compared to email spam and SPIT. Voice mail spam involves unsolicited messages left in a voice mail system (for example, the unsolicited messages left in an enterprise voice mail system, e.g., Cisco's Unity system). Voice mail spam is an especially expensive problem for users who retrieve messages from the voice mail system via cell phones or long distance calls. Voice mail spam causes users to spend real dollars (air time, per minute charges, toll free charges to enterprise etc) on messages they do not want to listen, not even to mention wasting their precious time.